1. Field of the Invention
The present invention relates to an apparatus and method for maintaining a constant extinction ratio of a laser diode of an optical transceiver, and more particularly, to an apparatus and method for maintaining a constant extinction ratio using correlations between a bias current and a modulation current.
2. Description of the Related Art
FIGS. 1A and 1B are graphs of a characteristic curve of a laser diode (LD) according to an increase in temperature. Referring to FIG. 1B, the characteristic curve of the LD shows that a threshold current Ith increases and the slope of the characteristic curve decreases as the temperature increases, or the deterioration of the performance of the LD.
When the slope of the characteristic curve decreases as stated above and the LD is operated using a bias current and a modulation current that are identical, optical power level and the extinction ratio are reduced causing deterioration in the transmission performance of the LD.
The extinction ratio indicates the ratio of two optical power levels 0 and 1 in a digital optical transport, and is expressed in dB.
Since a receiver can easily determine the optical power levels 0 and 1 as the extinction ratio increases, the extinction ratio is one of the important performance parameters that a transmitter must maintain constantly even if temperatures increase or there are changes in the environment.
FIGS. 2A and 2B are graphs of a characteristic curve of an LD with an automatic power control (APC). Referring to FIG. 2, most optical transceivers usually used in an optical communication system, are manufactured to maintain optical power through APC even when the slope of the characteristic curve of the LD decreases.
A thermistor or temperature sensor was used in order to compensate for a reduction of the extinction ratio of the LD, as shown in FIG. 3.
However, since the method of compensating for the reduction of the extinction ratio by sensing the change in the temperature and using the feedback circuit for changing the modulation current compensates for a change in the modulation current according to the change in the temperature using a linear function that has one slope, it is impossible to maintain the constant extinction ratio over the broad temperature range.